A. Technical Field
This invention relates to heat powered engines, and more particularly to a highly efficient form of heat powered, reciprocating-piston, harmonically acting engine having, in one embodiment, harmonic oscillator valves automatically controlling working fluid flow into and out of an expander at a resonant frequency, and in another embodiment, a shunt channel connecting a buffer chamber of the expander to the outlet of an expansion chamber of the expander, to minimize pressure perturbation in the engine fluidic circuit.
B. Description of the Related Art
Heat powered engines are known in which heat is supplied externally of the working cylinders rather than internally, in contrast to internal combustion engines. In prior art circuital flow-type (closed cycle) heat powered engines, a working fluid flows in a loop sequentially through a compressor, a heater, an expander, a cooler and finally back to the compressor. In an open cycle version, air is the working fluid and the ambient atmosphere performs the role of the cooler. Optionally, a heat interchanger transfers heat from the working fluid flowing between the expander and the cooler to the working fluid flowing between the compressor and the heater.
An early example of such a heat powered engine is described in U.S. Pat. No. 14690, entitled “Air Engine” by John Ericsson. A schematic illustration of this type of engine, but drawn with modernized mechanisms to facilitate comparison with the present invention, is shown in FIG. 1. This is an open cycle, heat powered engine having a single cylinder 57 with a single reciprocating piston dividing the internal cylinder volume into an expander chamber 54 and a compressor chamber 52. Incoming air 51 is drawn into compressor chamber 52 and raised in pressure, then sent to heater 53 and raised in temperature, then admitted to expander chamber 54 and dropped in pressure, and finally outgoing air 55 is released back to the ambient atmosphere. A heat interchanger 56 is provided to transfer some of the heat of the outgoing air to the pressurized air emerging from the compressor on its way to the heater. The arrows in FIG. 1 indicate the direction of flow of the air during the upstroke of the piston in this engine. However, a drawback of this single cylinder arrangement is the significant flow of heat from the high temperature expander chamber to the low temperature compressor chamber via the cylinder wall and the piston, which incurs a significant loss of thermal efficiency.
In U.S. Pat. No. 3,708,979 to Bush et al, entitled “Circuital Flow Hot Gas Engines,” an improved form of closed cycle, hot gas engine is described that provides separate cylinders for the expander and compressor, and thus avoids the “short circuit” flow of heat between the expander and compressor previously described. A schematic illustration of an engine arrangement similar to the Bush reference is shown in FIG. 2 having valves in the gas flow circuit which define four separate volumes (when all valves closed) of gas and which control the flow of gas through the four volumes. These four volumes include the volume in the expander, the volume in the compressor, the transport volume from the compressor exhaust to the expander intake via the heater, and the transport volume from the expander exhaust to the compressor intake via the cooler. At various phases of the engine cycle, different combinations of these four volumes are placed in fluidic communication. For example, FIG. 2 illustrates one particular phase in the engine cycle in which the volume within the expander, the transport volume that passes through the cooler, and the volume within the compressor are all contiguous. Arrows in this figure indicate the direction of the gas flow at this particular phase. In this manner, as suggested in the Bush reference, the valves in the gas flow circuit provide a means for isolating the portion of the gas mass involved in expansion and compression, from the portion of the gas involved in exchanging heat with heaters or coolers. Thus very efficient heat transfer can be achieved without attenuation of the pressure swings involved in gas expansion and compression inside of the working cylinders, in sharp contrast to the case with Stirling engines.
However, considering the variable rates of flow of the gas through such a circuit, the mass contained within each of the four distinguishable volumes varies through the engine cycle. As a result, pressure variations in the fluid circuit are produced that may be detrimental to the thermal efficiency of the engine. In order to minimize the detrimental effect of these pressure variations, the Bush reference teaches the use of header volumes, both at the expander inlet 58 and at the expander outlet 59. These header volumes, however, need to be substantially larger than the displacements of the compressor and expander. In rough approximation, in order to reduce the undesirable pressure deviations to the 1% level, the header volumes need to be approximately 100 times greater than the working cylinder volume throughput per cycle. Since the volume throughput associated with the high pressure side is much less than for the low pressure side, the header volume at the exit of the expander, in particular, entails a significant engine mass and volume penalty in order to achieve high efficiency.
Furthermore, the use of an expander cross head linkage 60 and a separate compressor cross head linkage 61, may make the frictional power loss in the system greater than necessary. Since the full power developed by the expander is transmitted to the crankshaft linkage 62, the bearing stresses may also be greater than necessary. Finally, the extra mechanisms associated with the extra cross head entail greater expense and less reliability than would be the case with a single cross head
In U.S. Pat. No. 1,038,805 to Webb, entitled “Hot Air Engine,” a tandem arrangement of working cylinders for air engines is disclosed. An illustration of an engine arrangement similar to the Webb reference is provided as FIG. 3 showing two separate cylinders for the compressor 63 and the expander 64, with the compressor piston connected to and sharing a common piston rod with the expander piston, but otherwise thermally isolated from each other to enable greater thermal efficiency. The use of a single cross head 65 to serve two cylinders is also advantageous as there are fewer rotating mechanisms. However, similar to the Bush patent, pressure swings in the fluid volume linking the two tandem cylinders of FIG. 3 can produce degradation in the thermal efficiency associated with the fact that the rate at which air is expelled from the compressor does not necessarily match the rate at which air is optimally ingested into the expander.
Furthermore, one of the most complicated and expensive features in the prior art of heat powered engines is the expander valve actuation mechanism. While the Webb reference teaches the use of automatic valves (such as reed valves, or the spring loaded poppet valves shown in FIG. 3) for controlling the flow of working fluid to the compressor, in contrast it teaches the use of “a slide or other valve, not shown, for controlling admission and exhaust” from the expander. FIG. 3 shows a sliding “Dee” valve 66 of a form well known in the art of steam engines. However, because of the sliding contact, such Dee valves must be lubricated to prevent undo friction, and are not able to function reliably at high speed and temperature. In contrast, poppet valves, such as those described in the Bush reference, avoid sliding contact, and are very highly developed in the field of internal combustion engines. Such poppet valves typically involve components such as cams, tappets, rockers and followers, as in conventional automobile engines, or pneumatic actuators, such as described by the Bush reference, or may involve electromagnetic actuators. With regard to the Bush reference in particular, at least three different means are disclosed by which the expander inlet valve may be opened automatically in response to either the increasing pressure within the expander cylinder as the expander piston approaches the top of the cylinder, or by actual contact with the expander piston itself. However, Bush does not teach how the expander inlet and outlet valves may be made to act fully automatically, as has long been known in the art for compressor valves.
In U.S. Pat. No. 6,062,181 to von Gaisberg et al, entitled “Arrangement for an electromagnetic valve timing control,” and in U.S. Pat. No. 6,302,068 to Moyer, entitled “Fast acting engine valve control with soft landing,” and in U.S. Pat. No. 6,394,416 to von Gaisberg, entitled “Device for operating a gas exchange valve,” the use of poppet valves partially actuated by springs is taught, with solenoids activated to open and/or to close the valves. In U.S. Pat. No. 5,058,538 to Erickson et al, entitled “Hydraulically propelled pneumatically returned valve actuator”, hydraulic and pneumatic activators are taught, instead of the solenoids used in the three previously mentioned cases. However, this prior art does not teach the use, or particular advantages of resonantly acting, harmonic oscillator valves in the expander of an external heat powered engine.
Thus there is a need to overcome the thermal inefficiency and pressure hysteresis factors associated with the known arrangements shown in the prior art, as well as overcome the other limitations of the prior art, including those associated with expander valves and their operation.